1. Field of the Invention
The present invention relates to a probe unit suitable for use with a scanning tunnel microscope (which will be hereinafter referred to as STM) intended to analyze the surface structure of a sample using the tunnel phenomenon of electrons.
2. Description of the Related Art
STM's provide extremely high resolution, and are capable of measuring concave and convex surface of a sample down to the level of atoms. STM's are also capable of measuring the state of electrons on the sample surface to be viewed. Such STM's have been developed as a device for observing surfaces of various samples. One of them is disclosed in U.S. Pat. No. 4,343,993. According to this STM, bias voltage is applied between the sample and the probe to generate tunnel current under such a condition that the conductive probe is moved toward the sample surface to be viewed with a distance of several nm or less interposed between them. Tunnel current detected by the probe which scans the sample surface is imaged to analyze the sample surface to be viewed.
The reason why this STM can have a resolution good enough to analyze the sample surface down to the level of atoms and to measure the state of electrons on the sample surface is that the tunnel phenomenon of electrons is used. The tunnel phenomenon of electrons means that when bias voltage is applied to two conductive matters materials (such as the metal probe and the conductive sample) under such a condition that they approach each other to a distance of several 10 .ANG. or less interposed between them, current (which is called tunnel current) flows between them. Usually, electrons are bound in a solid by binding energy which is called work function, and they cannot be forced outside the solid unless energy larger than this binding energy is added to them. However, an electron cloud envelopes the surface of the solid. When both of the probe and the sample surface are brought toward each other to tunnel region, several nm or less in length, their respective electron clouds overlap each other to thereby make their electrons freely movable. When bias voltage is applied to them under this state, tunnel current flows between them.
Tunnel current I can be expressed as follows: EQU I.varies.e.times.p(-k.multidot..phi..multidot.d)
wherein K represents a constant, .phi. an average of work functions of both of the probe and the sample, and d a distance (or tunnel region) between the probe and the sample.
As expressed by this formula, tunnel current I depends largely on distance (d) and the value of tunnel current is changed by a unit or more because of an atom which forms a concave or convex on the sample surface to be viewed. When tunnel effect is used, therefore, resolution relative to the sample surface can be significantly raised in the vertical direction of the sample surface.
The STM which uses tunnel effect can have the following advantages in addition to extremely high resolution.
1) Measurement relative to the sample surface can be conducted in atmospheric pressure (or air), gas, liquid, vacuum and at low temperature.
2) Atoms of the sample can be viewed not in reciprocal lattice space but in physical space.
3) Measurement can be conducted without contacting and damaging the sample.
4) The sample can be measured as it is without applying any specific process to the sample.
5) The surface physical property of the sample can be measured.
STMs are being applied to various fields, using these advantages.
The behavior of the STM will be described below.
The STM includes an actuator for adjusting the distance between the probe and the sample in direction Z (or direction of axis Z) and another actuator for adjusting the sample in directions (or directions of X and Y on plane) perpendicular to direction Z. The probe having a sharp tip is brought near the sample surface by the Z-direction actuator to such an extent that electron clouds respectively enveloping both of them slightly overlap each other. Voltage (or tunnel voltage) is applied between them to cause tunnel current to flow from the probe to the sample. The sample is moved in directions X and Y by the XY-direction actuator, while servo-operating the Z-direction actuator to hold the tunnel current constant, and the sample surface to be viewed is two-dimensionally scanned by the probe. Servo-voltage applied to the Z-direction actuator which servo-operates the probe is read and imaged to enable the sample surface to be observed. In other words, the probe scans the sample surface in the XY directions while the tunnel current is at a certain value set by the spacing between the probe and the sample surface. When the probe meets a stepped portion on the sample surface, tunnel current increases. In response, the probe is separated from the sample by the Z-direction actuator until tunnel current returns to the certain value. The amount of movement of the probe caused by the Z-direction actuator changes corresponding to concaves and convexes on the sample surface. The servo-voltage is read while repeating the scanning of the probe relative to the sample surface, and from it the image of the sample surface can be obtained.
In the case where the sample surface to be viewed is flat even when viewed on atomic scale, the probe is not particularly adjusted by the Z-direction actuator but tunnel current detected only when the probe two-dimensionally scans the sample surface may be imaged.
In the case where the sample is to be observed through the STM, the sample is first visually viewed by the operator to identify a point necessary to be observed, and this point of the sample surface is then observed in detail through the STM. Therefore, measurement relative to fine points difficult to be visually viewed that is, measurement relative to plural points on a grain of ceramics, comparison measurement relative to plural points on different grains of ceramics and observation relative to LSI patterns, gratings and pits on the compact disk cannot be conducted.
In the case where these fine points on sample surfaces are to be observed through an STM, the optical microscope is combined with the STM. The sample surface is viewed at a large area through the optical microscope to identify some points in this area of the sample surface, and these identified points are then observed in detail through the STM. Observation relative to the fine points on the sample surface can be thus conducted.
This STM observation makes it necessary for the probe unit, which serves as the probe, to be located in front of the objective lens of the optical microscope or between the objective lens and the sample to be viewed. However, the probe unit is optically opaque. This opaque probe unit therefore hinders the field of view of the optical microscope, thereby making it impossible to combine the STM with the optical microscope to observe fine points on the sample surface.